Generator question for elec electronic engineers

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[lw395]

If you were to short neutral to earth downstream of an RCD, it would not trip as soon as the power is turned on, assuming no load. This is because no fault current would exist.

You would find however that once you start to put a load downstream of the RCD, that it would disconnect, once the RCD senses a fault (imbalance between sum total of L and N respectively).

I disagree.
N and E each have a low impedance, it's common for them to have a different potential, which only needs to be very small to make 30mA flow through N to E, which is an imbalance as it's not flowing through L.
It is possible for it not to trip. Or only trip later when it's really inconvenient.

I would assume that the idea of connecting N to E 'in the consumer unit' assumes that the only 230V source is the generator. If there's a shore power/generator change over switch, the N to E connection needs to be on the generator branch of that. At the generator would make more sense to me.

 

[matthewriches]

I'm not saying that you should connect them (quite the opposite), merely that with the Earth and Neutral connected, RCD turned on and all loads turned off, the imbalance isn't there. Add some load, and it opens and will not reset until all load removed.

Not professional advice but take the front off a CU and try it. It doesn't happen until you add some load.

 

[PaulRainbow]

I'm not saying that you should connect them (quite the opposite), merely that with the Earth and Neutral connected, RCD turned on and all loads turned off, the imbalance isn't there. Add some load, and it opens and will not reset until all load removed.

Not professional advice but take the front off a CU and try it. It doesn't happen until you add some load.

It does not matter in the slightest whether the RCD will trip is there is no load, or not, having an installation that can only be used if there is no load is just plain ridiculous, is it not ?

My point was, you cannot have a PE-N connection at the consumer unit because the RCD will trip, why invent a scenario where it might not trip, no one is going to install a shore power system that has no loads ?

Claims that reverse polarity would cause a positive to earth short are incorrect, as are claims that if the RCD did not trip the MCBs would trip.

 

[lw395]

I'm not saying that you should connect them (quite the opposite), merely that with the Earth and Neutral connected, RCD turned on and all loads turned off, the imbalance isn't there. Add some load, and it opens and will not reset until all load removed.

Not professional advice but take the front off a CU and try it. It doesn't happen until you add some load.

That's basically only going to be the case if N and E are also connected together very close upstream of the consumber unit.
The voltage needed on N to get 30mA flowing is very small!

 

[VicS]

]

If you were to short neutral to earth downstream of an RCD, it would not trip as soon as the power is turned on, assuming no load. This is because no fault current would exist.

You would find however that once you start to put a load downstream of the RCD, that it would disconnect, once the RCD senses a fault (imbalance between sum total of L and N respectively).

I disagree.
N and E each have a low impedance, it's common for them to have a different potential, which only needs to be very small to make 30mA flow through N to E, which is an imbalance as it's not flowing through L.
It is possible for it not to trip. Or only trip later when it's really inconvenient.

I would assume that the idea of connecting N to E 'in the consumer unit' assumes that the only 230V source is the generator. If there's a shore power/generator change over switch, the N to E connection needs to be on the generator branch of that. At the generator would make more sense to me.

I'm not saying that you should connect them (quite the opposite), merely that with the Earth and Neutral connected, RCD turned on and all loads turned off, the imbalance isn't there. Add some load, and it opens and will not reset until all load removed.

Not professional advice but take the front off a CU and try it. It doesn't happen until you add some load.

It does not matter in the slightest whether the RCD will trip is there is no load, or not, having an installation that can only be used if there is no load is just plain ridiculous, is it not ?

My point was, you cannot have a PE-N connection at the consumer unit because the RCD will trip, why invent a scenario where it might not trip, no one is going to install a shore power system that has no loads ?

Claims that reverse polarity would cause a positive to earth short are incorrect, as are claims that if the RCD did not trip the MCBs would trip.

That's basically only going to be the case if N and E are also connected together very close upstream of the consumber unit.
The voltage needed on N to get 30mA flowing is very small!

I have just tried it. ...

But i dont know why I bothered so I have deleted the rest of the post

 

[PaulRainbow]

I have just tried it. ... It tripped ( no load) .... I knew it would because I've tried it before....... I said so earlier but you were too busy fighting and calling each other rude names to take any notice.

Most posts from #36 on were caused by your statement that having a PE-N link at the consumer unit would be wrong, because if the polarity were reversed it would short positive to earth and your follow up claim that if the RCD did not trip the MCB would. Your test above clearly proves that neither of these scenarios would be possible, because the real reason for not being able to have local PE-N links is that the RCD will trip.

Had you have accepted the mistake, the last couple of pages would not exist.

 

[VicS]

Most posts from #36 on were caused by your statement that having a PE-N link at the consumer unit would be wrong, because if the polarity were reversed it would short positive to earth and your follow up claim that if the RCD did not trip the MCB would. Your test above clearly proves that neither of these scenarios would be possible, because the real reason for not being able to have local PE-N links is that the RCD will trip.

Had you have accepted the mistake, the last couple of pages would not exist.

Absolute total rubbish.... Almost all the the posts from #36 onwards, apart from those whre you are insulting or being downright rude towards other contributors to the threda, were about wheter or not a PE to neutral link would cause the RCD to trip or wheter it would only do so if there is load on the system

Why do you not try to actually read what other people post rather than making up some garbage of your own.

By the way AC systms do not have a "positive" .............. Must get things correct to avoid confusing the world wide readership, or something like that, I seem to remember you saying!

 

[rogerthebodger]

By the way AC systms do not have a "positive" .............. Must get things correct to avoid confusing the world wide readership, or something like that, I seem to remember you saying!

Quite correct a single phase AC systems in the UK, South Africa and others have a line and neutral. The neutral is defined by the connection on one of the supply lines to the earth at the SUPPLY POINT. This is either the place where the electricity is produced (Generator, inverter) or a power transformer with electrically separated primary and secondary windings.

If there is no earth to one of the supply lines the system is a balanced supply. There are special balanced systems line a work site transformer that has a centre tap to earth so that the maximum voltage to earth is 55 Vac.

All of this if first year engineering degree stuff and is based on some of the fundamental laws of electricity.

This diagram was posted above and demonstrated the correct method of connection

The source in this diagram can be a generator, an inverter or and isolating transformer. The isolating transformer can be on board to prevent galvanic current or it can the a substation isolating transformer.

The ground spike or hull conductive plate is a secondary current path back to the neutral line at the source of the current.

Where a RCD is fitted on the installation side of the diagram. an alternate path for leakage current is required to there to be an imbalance in the current flow in the line and neutral wires.

This is one of the fundamental laws that you learn in first year electrics. Kirchoff's First Law the conservation of charge. Look it up

https://www.google.co.za/search?q=K...aw&aqs=chrome..69i57&sourceid=chrome&ie=UTF-8

The N-PE link could be fitted at the incoming terminals of the garage/consumer unit for a shore supply but its not really necessary as the N-PE connection be at the substation transformer unless you have an isolation transformer on board the if that transformer feeds the consumer unit directly that could ne a convenient place.

I have a setup with a single phase mains generator, An inverter as well as shore power selectable with a 4 position 2 pole switch. incoming earth line is connected to my steel hull via GI and the earth wiring is connected to all the appliances directly from the incoming earth NOT directly to the hull. The setup would be different if I had a isolating transformer.

 

[PaulRainbow]

By the way AC systms do not have a "positive" .............. Must get things correct to avoid confusing the world wide readership, or something like that, I seem to remember you saying!

Quite correct a single phase AC systems in the UK, South Africa and others have a line and neutral.

Until recently the live wire in the UK was called..... the live wire, someone decided we had to call it "line". It's variously referred to as live, line, positive, hot and no doubt some others. The "Earth" wire is variously referred to as Earth, ground, PE or CPC.

No matter which is used, people still know what is meant. Other than pedantry, it makes no difference.

 

[rogerthebodger]

\
No which is used, people still know what is meant. Other than pedantry, it makes no difference.

Paul that is precisely the problem not ALL people know that and when you deal with non technical people they get confused.

That someone who decided to call it line did that for a very good reason and that id to try to reduce/eliminate the confusion.

Its not just being pedantic its to avoid confusion in the minds of the non technical people.

 

[PaulRainbow]

The N-PE link could be fitted at the incoming terminals of the garage/consumer unit for a shore supply but its not really necessary as the N-PE connection be at the substation transformer unless you have an isolation transformer on board the if that transformer feeds the consumer unit directly that could ne a convenient place.

This is incorrect, as already extensively discussed in this thread. You cannot make the PE-N connection at the consumer unit in a shore power installation, it will trip the RCD.

 

[rogerthebodger]

This is incorrect, as already extensively discussed in this thread. You cannot make the PE-N connection at the consumer unit in a shore power installation, it will trip the RCD.

The RCD will only trip if there is an imbalance in the line and neutral connected to the RCD.

How will just fitting a N-PE link between line and neutral the incoming side of the consumer unit trip the RCD without any other fault.

Please give me a technical explanation not just unsubstiation statements.

BTW have a look of this

Hierarchy of Disagreement

We need to get away fro the lower 4 actions and get into the top 3 to have a meaningful discussion

 

[PaulRainbow]

The RCD will only trip if there is an imbalance in the line and neutral connected to the RCD.

How will just fitting a N-PE link between line and neutral the incoming side of the consumer unit trip the RCD without any other fault.

Please give me a technical explanation not just unsubstiation statements.

It will trip the shoreside RCD.

Look back at post #12, i said it there and several times since.

 

[rogerthebodger]

The RCD will only trip if there is an imbalance in the line and neutral connected to the RCD.

How will just fitting a N-PE link between line and neutral the incoming side of the consumer unit trip the RCD without any other fault.

Please give me a technical explanation not just unsubstiation statements.

/QUOTE]

It will trip the shoreside RCD.

Where does the imbalance of current in the shore side RCD come from with only a link in place.

Please analysis the current flows and current paths.

In my view a link in the consumer unit will cause the RCD in the consumer unit to tip when a line to earth short occurs. The shore side RCD will not see an imbalance so will not trip.

If there is a line earth short between the input to the consumer unit and the shore side RCD the shore side RCD will trip and the consumer unit RCD will not.

Equally if there is a short between the shopr side RCD and the main power supply distribution the main distribution RCD will trip.

If there is no RCD in the system and a short occurs the lowest rated overload upstream of the short will trip but will not protect and person in the path of the short as they would receive the full current load of the overload breaker.

When I lived in the UK I cannot recall having a RCD in my power supply mostly fuses. In South Africa I was surprised that MCB and RCD were fitted my house that is almost 45 years old and I left UK 36 years ago.

 

[PaulRainbow]

Where does the imbalance of current in the shore side RCD come from with only a link in place.

Please analysis the current flows and current paths.

I suggest you read back a few posts, all of the information is there, for me to re-type it would be a pointless waste of time.

 

[lw395]

Where does the imbalance of current in the shore side RCD come from with only a link in place.

Please analysis the current flows and current paths.

In my view a link in the consumer unit will cause the RCD in the consumer unit to tip when a line to earth short occurs. The shore side RCD will not see an imbalance so will not trip.
....

That assumes that the neutral, at the shore side RCD is at exactly zero volts relative to the ground connection there.
This is very often not the case!
The neutral will be grounded many metres away so the resistance won't be zero. There's also inductance.
If someone is drawing say 10A on another pontoon sharing some of the wiring there will be a voltage drop on the neutral.
Just looking for test limits on the web suggests as much as 15V on the neutral is possible?
But since the resistances are very low it will only take mV to get 30mA and trip any RCD where N and PE are shorted downstream of it.

Also the shoreside socket will ground PE at that point, to the local metalwork which is probably partly in the sea. There may even be a DC voltage difference to the GND at the substation, due to ground chemistry or even anodes on the pontoon....

 

[rogerthebodger]

That assumes that the neutral, at the shore side RCD is at exactly zero volts relative to the ground connection there.
This is very often not the case!
The neutral will be grounded many metres away so the resistance won't be zero. There's also inductance.
If someone is drawing say 10A on another pontoon sharing some of the wiring there will be a voltage drop on the neutral.
Just looking for test limits on the web suggests as much as 15V on the neutral is possible?
But since the resistances are very low it will only take mV to get 30mA and trip any RCD where N and PE are shorted downstream of it.

Also the shoreside socket will ground PE at that point, to the local metalwork which is probably partly in the sea. There may even be a DC voltage difference to the GND at the substation, due to ground chemistry or even anodes on the pontoon....

I hear what you are saying buy still don't get where the 30mA is flowing from and between which points 15V on the neutral are between.

 

[PaulRainbow]

I hear what you are saying buy still don't get where the 30mA is flowing from and between which points 15V on the neutral are between.

You said

The N-PE link could be fitted at the incoming terminals of the garage/consumer unit for a shore supply but its not really necessary as the N-PE connection be at the substation transformer unless you have an isolation transformer on board the if that transformer feeds the consumer unit directly that could ne a convenient place.

If the RCD will trip straight away, as some posters (including Vic, who says he has tested the theory twice), or as soon as a load exists, what is the point of such an installation ?

Can you explain to me, because i'm at a loss, what the purpose of such an installation would be ?